1. Field of the Invention
The present invention generally relates to methods and systems for measuring the properties of a formation through which a borehole passes. More particularly, the present invention relates to induction logging tools that measure the formation response to vertically- and horizontally-oriented dipoles. Still more particularly, the present invention relates to a method for combining the measured responses to achieve an induction log having reduced shoulder and borehole effects.
2. Description of the Related Art
The basic principles and techniques for electromagnetic logging for earth formations are well known. Induction logging to determine the resistivity (or its inverse, conductivity) of earth formations adjacent a borehole has long been a standard and important technique in the search for and recovery of subterranean petroleum deposits. In brief, the measurements are made by inducing electrical eddy currents to flow in the formations in response to an AC transmitter signal, and measuring the appropriate characteristics of a receiver signal generated by the formation eddy currents. The formation properties identified by these signals are then recorded at the surface as a function of the depth of the tool in the borehole. This record of the measurements is commonly termed xe2x80x9ca logxe2x80x9d, or more specifically, xe2x80x9can induction logxe2x80x9d of the formation.
It is well known that subterranean formations surrounding an earth borehole may be anisotropic with regard to the conduction of electrical currents. The phenomenon of electrical anisotropy is generally a consequence of either microscopic or macroscopic geometry, or a combination thereof, as follows.
In many sedimentary strata, electrical current flows more easily in a direction parallel to the bedding planes than it does in a direction perpendicular to the bedding planes. One reason is that a great number of mineral crystals possess a flat or elongated shape (e.g., mica or kaolin). At the time they were laid down, they naturally took on an orientation parallel to the plane of sedimentation. The interstices in the formations are, therefore, generally parallel to the bedding plane, and the current is able to easily travel along these interstices which often contain electrically conductive mineralized water. Such electrical anisotropy, sometimes called microscopic anisotropy, is observed mostly in shales.
Subterranean formations are often made up of a series of relatively thin beds having different lithological characteristics and, therefore, different resistivities. In well logging systems, the distances between the electrodes or antennas are great enough that the volume involved in a measurement may include several such thin beds. When individual layers are neither delineated nor resolved by a logging tool, the tool responds to the formation as if it were a macroscopically anisotropic formation. A thinly laminated sand/shale sequence is a particularly important example of a macroscopically anisotropic formation.
If a sample is cut from a subterranean formation, the resistivity of the sample measured with current flowing parallel to the bedding planes is called the transverse or horizontal resistivity xcfx81H. The inverse of xcfx81H is the horizontal conductivity "sgr"H. The resistivity of the sample measured with a current flowing perpendicular to the bedding plane is called the longitudinal or vertical resistivity, xcfx81v, and its inverse the vertical conductivity "sgr"V. The anisotropy coefficient xcex is defined as: xcex={square root over ("sgr"h/"sgr"v)}.
In situations where the borehole intersects the formation substantially perpendicular to the bedding planes, conventional induction and electromagnetic wave propagation well logging tools are sensitive exclusively to the horizontal component of the formation resistivity. This is a consequence of the induced currents flowing in horizontal planes in the absence of formation dip or well deviation. Indeed, regarding Galvanic devices, the lack of sensitivity to anisotropy is even more stringent due to the xe2x80x9cparadox of anisotropyxe2x80x9d, which states that any array of electrodes or sensors deployed along the axis of a wellbore in a vertical well is insensitive to the vertical component of resistivity, despite the intuitive expectation to the contrary.
However, it becomes possible to measure the vertical resistivity by orienting antenna coils away from the axis of the induction tool. An example of a commercial instrument that measures both horizontal and vertical resistivity is described by B. Kriegshauser, et al., describe this instrument in xe2x80x9cA new multicomponent induction logging tool to resolve anisotropic formationsxe2x80x9d, 41st Annual Logging Symposium, Society of Professional Well Log Analysts, paper D, pps. Jan 14, 2000. This instrument employs multiple multi-component coils (i.e. transmitter and receiver coils having axial and transverse orientations). Other tools designed to measure both horizontal and vertical resistivity are described in U.S. Pat. No. 4,302,723 entitled xe2x80x9cApparatus and method for determining dip and/or anisotropy of formations surrounding a boreholexe2x80x9d by J. Moran and in U.S. patent application Ser. No. 09/583,184, entitled xe2x80x9cMethod for Iterative Determination of Conductivity in Anisotropic Dipping Formationsxe2x80x9d and filed May 30, 2000, by inventors L. Gao and S. C. Gianzero.
Unfortunately, induction logging of both horizontal and vertical resistivities suffers from what is termed the xe2x80x9cshoulder effectxe2x80x9d. The true formation resistance is believed to vary as a function of depth in a fairly rectilinear fashion, i.e. the resistance changes discontinuously as one crosses boundaries between formation layers. However, because the tool measures the properties of a nonzero formation volume, the tool measurement actually varies gradually as the tool moves across a boundary. This is because some portion of formations on both sides of the boundary are within the measurement volume and consequently both contribute to the measurement until the tool is well past the boundary. In induction tools the measurement volume extends infinitely in all directions, with the effect of the formation resistance falling off exponentially with distance from the tool. The contribution of the formation outside the area of immediate interest produces a generally undesirable xe2x80x9csofteningxe2x80x9d of the induction log, i.e. the resistance changes gradually as boundaries are encountered. This effect is most evident when thin formation layers are considered. Where an ideal resistivity tool would show two discontinuities from the opposite boundaries of the thin layer, the actual log shows only a small bump between the boundaries. In effect, the xe2x80x9cshouldersxe2x80x9d of the ideal resistivity tool have been erased.
Also, an induction log of vertical resistivities tends to suffer from an inordinate borehole effect. That is, the borehole fluid and borehole geometry affect the measurement of the formation resistivity in an undesirable way. A method of induction logging that reduces both the shoulder effect and the borehole effect would be very desirable.
Accordingly, there is disclosed herein a method of enhancing the vertical resolution of an induction tool, in a manner that may advantageously also reduce undesirable borehole and xe2x80x9cnegative resistivityxe2x80x9d effects. In one embodiment, the method comprises: a) obtaining a vertical magnetic dipole (VMD) response signal from a transmitter-receiver array of antenna elements having magnetic dipoles oriented parallel to a tool axis; b) obtaining a horizontal magnetic dipole (HMD) response signal from a transmitter-receiver array of elements having magnetic dipoles oriented perpendicular to the tool axis; and c) combining the VMD and HMD response signals to obtain a combination response signal. When the relative weights of the VMD and HMD response signals are set as described herein, the combination response signal (and any log calculated therefrom) has a narrow, substantially rectilinear, vertical measurement profile. Further, the combination response signal is relatively insensitive to the negative borehole contribution intrinsic to HMD.